Transparent conductors are necessary components for smartphones, organic photovoltaics (OPVs), organic light emitting diodes (OLEDs), flat panel displays, and touch sensors. Indium tin oxide (ITO) is the chief material utilized for this purpose due its low sheet resistance (Rs=10 Ωsq-1) at high transmittance values (>90% T). However, indium is a scarce (less than 0.05 ppm in the Earth's crust) and expensive (˜$600 kg-1) starting material and the brittle ceramic nature limits its use in flexible applications. Additionally the vapor phase sputtering process required to fabricate ITO electrodes involves slow linear coating rates that decrease for thicker films resulting in higher final costs. Since solution-phase coating processes do not have to sacrifice speed for thickness and offer coating speeds more than 100 times faster than sputtering methods, an ITO alternative that can be coated from solution without compromising optoelectronic performance can be beneficial to the industry.
Several contenders have emerged as promising candidates to supplant ITO such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), carbon nanotubes (CNTs), and graphene, but only metal nanowires are both solution-coatable and demonstrate comparable performance to ITO. Silver nanowires (Ag NWs) have become a frontrunner as they are immediately conductive after coating and the resulting films have high thermal and oxidation stability thus making them attractive as the transparent conducting layer in devices such as OPVs or OLEDs. But silver is even more expensive than indium (˜$750 kg-1) with a similar abundance. Copper on the other hand is only 6% less conductive than silver (ρCu=1.68×10-8 Ωm, ρAg=1.59×10-8 Ωm) but more than 100 times cheaper and 1000 times more abundant. Thus, much attention has been focused on transitioning to more cost-effective copper nanowire based transparent conducting films.
Further, hydrogen annealing at high temperature (200° C.) can burn off the organic material, reduce the surface oxides, and sinter the nanowires together to render the films conductive, but this method can be dangerous, unsuitable for large-scale manufacturing, and does not inhibit future copper oxidation. Recently, a low-temperature solution-based approach has proven to produce similar optoelectronic performance for Cu nanowire films as H2 gas annealing by removing the organic material and etching away the native oxides though carboxylic acid treatment, but this method still fails to protect the nanowires from further oxidation thus hindering their long-term use.
In order to hurdle this barrier, there have been numerous efforts to prevent Cu nanowire oxidation without degrading the optoelectronic properties of the film. Various groups have attempted encapsulating Cu nanowires in an overcoat, such as a grapheme composite or aluminum-doped zinc oxide (AZO), or embedding the nanowires in a plastic. However, these methods require H2 gas or are not scalable. Scalable procedures for depositing Ni as a protective shell on the Cu nanowires were explored, but these methods decreased the transmittance of the nanowire films and thus the overall performance of the networks. To overcome this, Zn, Sn, and In have been electrodeposited on films of copper nanowires and subsequently oxidized to form transparent metal oxide shells that protected the Cu nanowires from oxidation without affecting the electrode performance. However, this technique is also not scalable and the deposited material has to be chemically altered through an additional step in order to regain transparency.
While Cu nanowires are a seemingly enticing solution to many of the problems facing ITO, there are still factors limiting their widespread use. Accordingly, there is a need for improved techniques for producing copper nanowires and other types of nanostructures.